"It is the freezing of water," said Mr. Hume. "In the process of crystallization one hundred and forty degrees of latent heat become sensible." "And this," continued Mr. Wilton, "is no inconsiderable matter. Every pound of water frozen upon the surface of our lakes and rivers, every pound of water frozen in the wet earth, every pound of water frozen as snow or sleet in the air, gives out as much heat as would boil an equal amount of water at seventy-two degrees. Have you never heard of setting tubs of water in cellars to keep vegetables from freezing?" "I have," replied Peter. "I visited my grandfather two years ago, and his cellar sometimes froze. I asked him why he put tubs of water in his cellar, but he could not tell me, only he said that he knew that tubs of water in his cellar did keep his vegetables from being nipped with the frost." "Can you tell us, Peter, why tubs of water set in a cellar should have this effect?" "I suppose that when the water begins to freeze it begins to give out its latent heat." "That is one part of the reason. The water is drawn from the well at perhaps fifty degrees; it must lose eighteen degrees of heat before it begins to freeze, and all the heat which the water loses the air of the cellar gains. And then, as you said, as soon as the water begins to freeze latent heat begins to become sensible. Every pound of water frozen sets free heat enough to raise a pound of water through one hundred and forty degrees. But why do not the vegetables begin to freeze as soon as the water?" "I don't know." "Water holding salt or other minerals in solution freezes at a lower temperature than pure water. For this reason the juices of vegetables and fruits and the sap of trees may be cooled below thirty-two degrees without freezing. On this account the water set in cellars tends to prevent vegetables from freezing; the water begins to freeze at thirty-two degrees, while potatoes and turnips may be cooled a little lower than thirty-two degrees without harm. In this manner the buds of trees are sometimes warmed and protected by the coating of ice which forms around them. The drops of water, falling through the sleety air, touch upon the twigs of trees and freeze upon them, an icy coat embracing them all around. In freezing, the water gives out one hundred and forty degrees of heat, a part of which goes to the air and a part to the twig." "This reminds me," said Ansel, "of what the Irishman said on being told that snow contains heat, that 'it would be a blessed thing for the poor if one could tell how many snowballs it would take to boil a tea-kettle.'"